This study investigated the rainfall patterns, spatial variability, surface runoff generation and dam requirements in the southwestern region of Saudi Arabia. The region was divided into four areas Asir, Jazan, Al-Baha and the Red Sea Coast. Surface runoff was estimated for eight scenarios considering the runoff coefficients of 0.05–0.70, resulting in 203–2,835 million cubic meters (MCM) of runoff per year in this region. The runoff in the Asir, Jazan, Al-Baha and the Red Sea Coast were estimated to be in the ranges of 88–1,230, 53–738, 32–443 and 30–425 MCM per year, respectively. The capacities of the existing dams in Asir, Jazan and Al-Baha are approximately 373, 194 and 31 MCM, respectively, while the coast does not have any dam. A significant fraction of runoff is likely to be lost in each scenario of assessment. Water resources may be augmented through construction of new dams and/or wells in appropriate locations. However, better understanding is advisable on locations, water availability, surface evaporation in wadies and reservoirs, accumulation of solids in dam/reservoirs, hydraulic conductivity, economic burdens and national policy.